diff --git a/packages/opencode/script/schema.ts b/packages/opencode/script/schema.ts
index 0aa4e2068efe..e467c2964e80 100755
--- a/packages/opencode/script/schema.ts
+++ b/packages/opencode/script/schema.ts
@@ -6,9 +6,9 @@ import { TuiConfig } from "@opencode-ai/tui/config"
 import { Schema } from "effect"
 
 type JsonSchema = Record<string, unknown>
-const MODEL_REF = "https://models.dev/model-schema.json#/$defs/Model"
+export const MODEL_REF = "https://models.dev/model-schema.json#/$defs/Model"
 
-function generateEffect(schema: Schema.Top) {
+export function generateEffect(schema: Schema.Top) {
   const document = Schema.toJsonSchemaDocument(schema)
   const normalized = normalize({
     $schema: "https://json-schema.org/draft/2020-12/schema",
@@ -50,13 +50,15 @@ function normalize(value: unknown): unknown {
   return schema
 }
 
-function restoreModelRefs(value: unknown, key?: string): unknown {
+export function restoreModelRefs(value: unknown, key?: string): unknown {
   if (Array.isArray(value)) return value.map((item) => restoreModelRefs(item))
   if (!isRecord(value)) return value
 
   const schema = Object.fromEntries(Object.entries(value).map(([name, item]) => [name, restoreModelRefs(item, name)]))
   if ((key === "model" || key === "small_model") && schema.type === "string") {
-    return { ...schema, $ref: MODEL_REF }
+    const { type: _, ...rest } = schema
+    // models.dev Model is a closed enum; sibling $ref rejects custom ids.
+    return { ...rest, anyOf: [{ type: "string" }, { $ref: MODEL_REF }] }
   }
   return schema
 }
@@ -65,13 +67,15 @@ function isRecord(value: unknown): value is JsonSchema {
   return typeof value === "object" && value !== null && !Array.isArray(value)
 }
 
-const configFile = process.argv[2]
-const tuiFile = process.argv[3]
+if (import.meta.main) {
+  const configFile = process.argv[2]
+  const tuiFile = process.argv[3]
 
-console.log(configFile)
-await Bun.write(configFile, JSON.stringify(generateEffect(ConfigV1.Info), null, 2))
+  console.log(configFile)
+  await Bun.write(configFile, JSON.stringify(generateEffect(ConfigV1.Info), null, 2))
 
-if (tuiFile) {
-  console.log(tuiFile)
-  await Bun.write(tuiFile, JSON.stringify(generateEffect(TuiConfig.Info), null, 2))
+  if (tuiFile) {
+    console.log(tuiFile)
+    await Bun.write(tuiFile, JSON.stringify(generateEffect(TuiConfig.Info), null, 2))
+  }
 }
diff --git a/packages/opencode/test/script/schema.test.ts b/packages/opencode/test/script/schema.test.ts
new file mode 100644
index 000000000000..0868f1fdece7
--- /dev/null
+++ b/packages/opencode/test/script/schema.test.ts
@@ -0,0 +1,121 @@
+import { describe, expect, test } from "bun:test"
+import { ConfigV1 } from "@opencode-ai/core/v1/config/config"
+import { generateEffect, MODEL_REF, restoreModelRefs } from "../../script/schema"
+
+const CUSTOM = "internal/Qwen/Qwen3-Coder-30B-A3B-Instruct"
+const KNOWN = "anthropic/claude-2"
+const KNOWN_MODELS = [KNOWN] as const
+
+const ISSUE_EXAMPLE = {
+  $schema: "https://opencode.ai/config.json",
+  provider: {
+    internal: {
+      npm: "@ai-sdk/openai-compatible",
+      models: {
+        "Qwen/Qwen3-Coder-30B-A3B-Instruct": {},
+      },
+    },
+  },
+  model: CUSTOM,
+}
+
+function isRecord(value: unknown): value is Record<string, unknown> {
+  return typeof value === "object" && value !== null && !Array.isArray(value)
+}
+
+/**
+ * JSON Schema 2020-12: `$ref` is applied together with sibling keywords.
+ * Stub models.dev `$defs/Model` as a closed enum so this test does not fetch.
+ */
+function acceptsModelId(schema: unknown, value: string, known: readonly string[]): boolean {
+  if (!isRecord(schema)) return false
+  if (Array.isArray(schema.anyOf)) {
+    return schema.anyOf.some((option) => acceptsModelId(option, value, known))
+  }
+  const refOk = schema.$ref !== MODEL_REF || known.includes(value)
+  const typeOk = schema.type === undefined || schema.type === "string"
+  if (schema.$ref === MODEL_REF || schema.type === "string") return refOk && typeOk
+  return false
+}
+
+function collectModelIdSchemas(value: unknown, key?: string, out: unknown[] = []): unknown[] {
+  if (Array.isArray(value)) {
+    for (const item of value) collectModelIdSchemas(item, undefined, out)
+    return out
+  }
+  if (!isRecord(value)) return out
+  if ((key === "model" || key === "small_model") && isModelIdSchema(value)) out.push(value)
+  for (const [name, item] of Object.entries(value)) collectModelIdSchemas(item, name, out)
+  return out
+}
+
+function isModelIdSchema(schema: Record<string, unknown>): boolean {
+  if (schema.type === "string") return true
+  if (schema.$ref === MODEL_REF) return true
+  if (Array.isArray(schema.anyOf)) {
+    return schema.anyOf.some((item) => isRecord(item) && isModelIdSchema(item))
+  }
+  return false
+}
+
+const modelFields = {
+  type: "object",
+  properties: {
+    model: {
+      type: "string",
+      description: "Model to use in the format of provider/model, eg anthropic/claude-2",
+    },
+    small_model: {
+      type: "string",
+      description: "Small model to use for tasks like title generation in the format of provider/model",
+    },
+    agent: {
+      type: "object",
+      additionalProperties: {
+        type: "object",
+        properties: {
+          model: { type: "string" },
+        },
+      },
+    },
+    command: {
+      type: "object",
+      additionalProperties: {
+        type: "object",
+        properties: {
+          model: { type: "string" },
+        },
+      },
+    },
+  },
+}
+
+describe("restoreModelRefs", () => {
+  test("accepts custom provider/model from the published schema example", () => {
+    const schema = restoreModelRefs(modelFields)
+    const fields = collectModelIdSchemas(schema)
+    expect(fields.length).toBe(4)
+    for (const field of fields) {
+      expect(acceptsModelId(field, ISSUE_EXAMPLE.model, KNOWN_MODELS)).toBe(true)
+    }
+  })
+
+  test("still accepts a normal models.dev id as a string", () => {
+    const schema = restoreModelRefs(modelFields)
+    for (const field of collectModelIdSchemas(schema)) {
+      expect(acceptsModelId(field, KNOWN, KNOWN_MODELS)).toBe(true)
+    }
+  })
+})
+
+describe("generated config schema", () => {
+  test("does not reject custom provider/model on Config.model and nested model fields", () => {
+    const schema = generateEffect(ConfigV1.Info)
+    const fields = collectModelIdSchemas(schema)
+    expect(fields.length).toBeGreaterThanOrEqual(4)
+    for (const field of fields) {
+      expect(acceptsModelId(field, CUSTOM, KNOWN_MODELS)).toBe(true)
+      expect(acceptsModelId(field, KNOWN, KNOWN_MODELS)).toBe(true)
+    }
+  })
+})